This invention relates to an improved precision measuring apparatus which defines a measuring interval sealed from environmental influences.
The term "measuring interval" is used herein to denote a segment of space in which a measurement determining or reading element of a measuring apparatus can move.
Measuring instruments having a member movable along a measuring interval are well known in the art. Among such instruments can be counted a laser interferometer having a movable reflector, and an instrument for determining the deviation of the path of a movable member from a straight line defined by a laser beam. Such an instrument can also take the form of a graduating machine for producing measuring grids having fine division fields. The instrument can also be a digital length-measuring apparatus in which the scale and the scanning unit are shielded against environmental influences. The shielding of the measuring interval assures that environmental influences on measurement will remain substantially unvarying.
West German OS No. 21 13 477 discloses a laser interferometer having the measuring interval enclosed in a shielding evacuated housing. The housing has a slit for the admission of a carrying member which carries the optical reflector. The slit is sealed by means of flexible sealing lips. The disadvantage of this arrangement is that the vacuum in the housing can be maintained only by continuous evacuation requiring relatively high capacity vacuum pumps.
To maintain a vacuum inside the housing, West German PS No. 24 21 371 proposes to close the slit in the shielding housing of an interferometer by means of a pair of flexible sealing lips which extend across the slit, one lip from each side of the slit, and which contact each other over the slit along the length of the slit to form a "roof" over the slit. The space around the sealing lips is filled with vacuum oil. In this arrangement only the relatively viscous oil can attempt to leak through whatever small leaks may exist in the sealing lips, making the housing more nearly airtight. When used in conjunction with the interferometer disclosed in OS No. 21 13 477, this liquid sealing arrangement has the advantage that evacuation of the housing need be repeated less often and consequently can be accomplished with vacuum pumps having a relatively lower capacity. However, this apparatus requires the addition of an oil pump for pumping the oil which leaks into the housing from the housing back to the sealing lips.
As an alternative way of sealing gaps in the housing sealing elements, West German PS No. 24 60 406 teaches sealing the gaps with a magnetic fluid and holding the fluid in place by means of a magnetic field.
However, in high-precision measuring instruments, such as the laser interferometer discussed above, problems arise due to the friction between the sealing elements and the carrying member which must pass through them. The carrying member transmits deformations caused by the friction to the measuring reflector, and this can result in measurement error. Since measurements may be in the range of the wavelength of light, even slight forces exerted on the carrying member can influence the measurements.